Polyester resins, polyethylene terephthalate (PET) and its copolyesters, are widely used to produce rigid packaging, such as two-liter soft drink containers. Polyester packages produced by stretch blow molding possess high strength and shatter resistance, and have excellent gas barrier and organoleptic properties as well. Consequently, such plastics have virtually replaced glass in packaging numerous consumer products (e.g., such as carbonated soft drinks, fruit juices, and peanut butter).
In conventional techniques of making bottle resin, polyethylene terephthalate or its copolyesters are polymerized in the melt phase to an intrinsic viscosity of about 0.6 deciliters per gram (dl/g). The polyethylene terephthalate is then polymerized in the solid phase to achieve a higher intrinsic viscosity that promotes bottle formation.
As will be understood by those having ordinary skill in the art, polyethylene terephthalate is typically converted into a container via a two-step process. First, an amorphous bottle preform is produced from bottle resin by melting the resin in an extruder and injection molding the molten polyester into a preform. Such a preform usually has an outside surface area that is at least an order of magnitude smaller than the outside surface of the final container. The preform is reheated to an orientation temperature that is typically 30.degree. C. above the glass transition temperature. The reheated preform is then placed into a bottle mold and, by stretching and inflating with high-pressure air, formed into a bottle. Those of ordinary skill in the art will understand that any defect in the preform is typically transferred to the bottle. Accordingly, the quality of the injection-molded preform is critical to achieving commercially acceptable bottles.
Conventional polymerization techniques rely primarily on melt-phase polymerization to produce polyester resins that facilitate efficient preform molding. Melt phase polymerization, however, is relatively expensive as compared to solid state polymerization (SSP). In particular, melt-phase polymerization requires a higher capital investment than does solid state polymerization.
To reduce the costs associated with preparing bottle resins, techniques have been developed to emphasize polymerization in the solid phase rather than the melt phase. For example, there are several patents assigned to DuPont that disclose modified polyethylene terephthalate compositions and methods for preparing the same. See, e.g., U.S. Pat. Nos. 5,510,454; 5,532,333; 5,540,868; 5,633,018; 5,714,262; 5,744,074; and 5,830,982. These patents especially disclose polyethylene terephthalate compositions having large crystallite sizes.
For example, U.S. Pat. No. 5,510,454 describes modified and unmodified polyethylene terephthalate prepolymer having a degree of polymerization of about 5 to about 35 (ie., an intrinsic viscosity ranging between about 0.10 dl/g and 0.36 dl/g), an average apparent crystallite size of 9 nm or more, and a melting point of 270.degree. C. or less. The related U.S. Pat. No. 5,714,262 further discloses a high molecular weight polyethylene terephthalate composition polymerized in the solid phase from a low molecular weight, large crystallite prepolymer, such as that disclosed by the '454 patent. Both the '454 patent and the '262 patent teach that polyethylene terephthalate can be modified by up to 10 mole percent comonomers--but preferably less than 5 mole percent--provided that the crystallization behavior of the polyester is substantially the same as unmodified polyethylene terephthalate.
Accordingly, these DuPont patents teach away from modified polyethylene terephthalate compositions that have substantially different crystallization behavior from those of "homopolymer" polyethylene terephthalate. This is critical because by embracing only polyesters that behave like homopolymer polyethylene terephthalate, the combined DuPont teachings yield high molecular weight copolyester resins that not only have large crystallite sizes, but also high crystallinity fraction. Polymer melt theory suggests that this combination causes high haze point temperatures. As will be understood by those of ordinary skill in the art, haze point is the temperature at which large, light scattering crystallites are present in the preform. This complicates the conventional processing of resins produced according to the DuPont teachings.
U.S. Pat. No. 4,165,420, which is assigned to Goodyear, discloses low molecular weight polyester prepolymer in the form of spherical beads that can be polymerized in the solid state to yield a high molecular weight resin. The prepolymer has an intrinsic viscosity of between about 0.1 dl/g and 0.35 dl/g. In accordance with this Goodyear patent, to achieve discrete spherical beads between 100 and 250 microns by employing either spray congealing or atomization requires that the solid state polymerization begin at an intrinsic viscosity of below 0.25 dl/g. The '420 patent also results in prepolymer having relatively large crystallite sizes.
Similarly, U.S. Pat. No. 4,755,587 and its continuation-in-part, U.S. Pat. No. 4,876,326, both of which are also assigned to Goodyear, disclose a method for producing high molecular weight polyester resins from low molecular weight prepolymers. In particular, the '587 patent discloses the solid state polymerization of polyester prepolymers in the form of porous pills. These prepolymers have an initial intrinsic viscosity between about 0.15 dl/g and 0.7 dl/g--preferably less than 0.3 dl/g--for a time sufficient to yield a high molecular weight polyester resin. The '587 patent also describes that a final intrinsic viscosity of at least 0.65 dl/g is desirable, and preferably an intrinsic viscosity of at least 0.7 dl/g. While the '587 patent discloses that the invention is applicable to virtually any polyester that can be solid state polymerized, it explains that the most common kind of polyesters to be solid state polymerized using the technique have about 75 mole percent of their acid component provided by aromatic dicarboxylic acids.
In general, the cited Goodyear patents disclose modified polyesters having both high and low molecular weight, as well as solid state polymerization methods that employ copolyester prepolymers. These patent disclosures, however, fail to teach the present method for preparing copolyester bottle resins that have excellent melt processing characteristics, specifically a low haze point temperature. In particular, these Goodyear patents teach away from using conventional pellets and instead employ very fine spherical beads or porous pills (i.e., less than 1 mm). For example, Goodyear's disclosed spray-congealing method produces spherical polyethylene terephthalate particles in the 100-200 nm range when the intrinsic viscosity is less than about 0.25 dl/g. Goodyear's relatively greater surface area per weight of the fine particles presumably promotes faster solid phase polymerization, albeit at the cost of larger crystallite sizes. In this regard, the heat treatment during the particle formation as taught by the aforementioned U.S. Pat. No. 4,165,420 results in crystallite sizes greater than about 9 nm. These Goodyear patents, however, fail to appreciate that solid state polymerizing prepolymer having a relatively large average crystallite size will result in resins that possesses unacceptably high melt temperatures.
In summary, the prior art discloses solid state methods of polymerizing low molecular weight polyester prepolymers to achieve high molecular weight polyester compositions. These methods, however, yield polyester compositions that possess unacceptably high haze points. Processing such polyester compositions through preform molding equipment at conventional temperature settings results in hazy bottles. Consequently, preform equipment must be operated at higher temperatures. This requires more cooling time, which slows process throughput as compared to conventional processes. Moreover, higher preform molding temperatures lead to high levels of polyethylene terephthalate decomposition products, such as acetaldehyde and color bodies. Therefore, there is a need for a high molecular weight copolyester bottle resin that can be polymerized primarily in the solid phase, and yet possesses excellent melt processing characteristics.